Vasopressin (AVP) antagonists represent a novel therapeutic class for the treatment of depression. The potential utility of these compounds has emerged from observations in depressed individuals, findings in animal models, and an understanding of changes in hypothalamic-pituitary-adrenal (HPA) axis regulation under chronic stress. This Fast Track proposal seeks support for the identification of novel mixed vasopressin 1a/1b (V1a/V1b) receptor antagonists and the preclinical development of recently discovered molecules in this class that already demonstrate excellent biological activity in vitro. The scientific basis for mixed V1a/V1b antagonists as a pharmacotherapy for depression includes: 1) the neuroadaptation and dysregulation of HPA function that accompanies chronic stress in affected humans and in animal models of depression, 2) recognition that AVP, not CRF, drives HPA function associated with chronic psychological stress, and 3) the localization of V1a and V1b receptors in regions involved in the control of social behaviors and HPA axis regulation (V1a in limbic system; V1b in limbic system and anterior pituitary). The initial development of these mixed antagonists to date has been supported by private sector venture funding. SBIR Fast Track support will enable essential preclinical development work that will advance candidate molecules to the stage where bulk synthesis and IND-enabling toxicology can be undertaken. Bringing candidates to this status will accelerate commercialization opportunities by significantly enhancing the likelihood of additional private financial investment or a co-development partnership structure with a major pharmaceutical house.